Various devices are known in the prior art for realizing a full color video picture of a target situated within a remote cavity. Most devices of this type employ an external light source conveyed to the image head by fiber optic bundles together with a solid state image sensor and lens system positioned in the distal end of the insertion tube of a borescope/endoscope, referred to herein as a probe, connected to an external video display. A particularly compact head including a light source and solid state image sensor lens system of this type is shown in U.S. Pat. No. 4,491,865 (Danna et al.).
Generally, in systems of this type, the fibre optic illumination bundle and the image sensor and optical system are disposed side by side in the end of a small insertion tube adapted to be inserted in cavities for viewing objects therein. The light provided by the fiber optic bundle has a field of view slightly displaced from the optical field of view of the image sensor; but generally overlapping sufficiently to provide an effective field of vision for the device. The image detected by the image sensor is displayed on a video screen and varies in magnification, apparent size, and detail, depending upon how close the end of the insertion tube carrying the lens system is from the object being viewed. Devices of this type typically have a depth of field from an eighth of an inch (3 mm) to something over four inches (100 mm). The displayed magnification decreases as the distance between the probe tip and the object being viewed increases.
Attempts to measure an object in the image on the video display to determine the size of the object being viewed typically rely on either placing a known scale adjacent to the object to be measured for a comparison measurement, or providing a physical standoff over the lens on the end of the probe insertion tube, at which point the magnification is known and the end of the probe is adjusted until it just touches the object to be viewed at the standoff. With this known magnification, the image can be measured on the screen and the precise size determined. A related method uses optics having a very narrow depth of field and an adjustable focal point. Feedback from the focal point adjustment is used to determine the distance to the in-focus-object and therefore the magnification of the object as viewed on the screen. This magnification is then used to perform measurements.
Another measuring system is disclosed in U.S. Pat. No. 4,980,763 (Lia) which measures objects viewed in a borescope by creating an auxiliary structure in the image, such as a shadow, which is projected onto the object so that its position in the video image changes in proportion to the distance of the image sensing head from the object.
U.S. Pat. No. 5,070,401 (Salvati et al.) discloses a 3-D video measurement system in which the depth or thickness of an object is determined along with its length and width. This system relies on the shadow method of the Lia patent to make its 3-D measurements. Although the method works well, it is difficult to achieve optimal shadow positioning and identification in some applications.
Using stereo images for 3-D measurements is becoming popular, U.S. Pat. No. 5,522,789 (Takahashi) discloses a stereo endoscope which includes a pair of objective optical systems, a pair of relay optical systems, an imagery optical system having a single optical axis, and a pair of imaging devices. U.S. Pat. No. 5,860,912 (Chiba) discloses a stereoscopic-vision endoscope system which uses two objective lens systems to provide separate images. The independent lens trains have diverging images so that, even at infinity, the views never overlap 100%. The separate lens trains also create the condition that the right image is displayed on the left side of the monitor and the left image on the right. The separate lens trains are typically very long, on the order of 15-20 mm for a 6 mm diameter probe, thus making the head of the probe very long. U.S. Pat. No. 6,184,923 (Miyazaki) uses a plurality of optical systems in a first detachable tip adapter and a single optical system in a second detachable tip adapter with an endoscope having no optics adjacent to the imager to obtain multiple overlapping fields of view. This approach also typically yields a long distal-tip length and requires a complex attachment mechanism.